2.3 Adjuvants & Tank Mixing Compatibility

Adjuvants, Spray Solution Dynamics, and Tank Mixing Protocols

Core Principle: An adjuvant is any chemical substance added to a pesticide spray mixture to enhance product performance or alter physical spray properties. Adjuvants possess no standalone pesticidal activity and must be used strictly in accordance with pesticide label instructions.

---

1. Adjuvant Classifications & Functional Mechanisms

Adjuvants are divided into two major operational groups:

1. Activator Adjuvants: Directly enhance the biological performance, foliar wetting, penetration, or retention of the active ingredient (e.g., surfactants, stickers, penetrants, crop oils).
2. Utility Adjuvants (Spray Modifiers): Alter the physical characteristics of the spray solution to improve tank handling, mixing, or drift control without directly increasing biological activity (e.g., buffers, defoamers, drift control agents, compatibility agents).

+-----------------------------------------------------------------------------------+
|                             ADJUVANT CLASSIFICATIONS                              |
+-----------------------------------------+-----------------------------------------+
|          ACTIVATOR ADJUVANTS            |           UTILITY ADJUVANTS             |
|  (Enhance Biological Efficacy)          |  (Modify Spray Tank / Solution Dynamics)|
+-----------------------------------------+-----------------------------------------+
| • Surfactants (NIS, Organosilicones)    | • Buffers & Acidifiers (pH Control)     |
| • Spreaders & Stickers                  | • Drift Control / Deposition Aids       |
| • Crop Oil Concentrates (COC / MSO)     | • Anti-Foaming / De-Foaming Agents      |
| • Penetrants                            | • Compatibility Agents (Fertilizer Mix) |
+-----------------------------------------+-----------------------------------------+

---

2. Detailed Adjuvant Categories

Surfactants (Surface-Active Agents)

• Physical Mechanism: Water possesses a high natural surface tension (~72 dynes/cm) due to internal hydrogen bonding, forcing spray droplets to remain spherical beads that bounce or roll off waxy plant cuticles. Surfactants contain both a hydrophilic (water-loving) head and a lipophilic (oil-loving) tail. They position themselves at the droplet surface, disrupting hydrogen bonds and dropping surface tension to 25–35 dynes/cm, allowing droplets to spread out into a thin, continuous liquid film.
• Nonionic Surfactants (NIS): Contain no electrical charge. They are chemically neutral, stable in hard water, compatible with virtually all systemic herbicides and fungicides, and represent the most widely used foliar surfactant.
• Anionic Surfactants: Carry a negative electrical charge. Primarily used in commercial formulation blending rather than field tank mixes.
• Cationic Surfactants: Carry a positive electrical charge. Highly phytotoxic to plant tissues; restricted to non-crop total-vegetation burn-down.
• Organosilicone Surfactants ("Super-Spreaders"): Silicon-based surfactants that lower surface tension below 22 dynes/cm. They cause stomatal flooding (forcing spray solution directly into plant leaf pores) but increase runoff risks if applied past the point of foliar runoff.

Spreaders and Stickers

• Spreaders: Increase the total surface area of leaf contact by flattening spray droplets.
• Stickers: Substances (pine resins, latex polymers, vegetable oils) that physically glue the pesticide particles to the foliage surface. They prevent wash-off from rainfall, heavy morning dew, and overhead irrigation, while reducing UV degradation.
• Spreader-Stickers: Pre-blended commercial adjuvants delivering both wetting expansion and wash-off resistance.

Penetrants & Crop Oil Concentrates (COC / MSO)

• Composition: Crop Oil Concentrates contain 80% to 85% phytobland petroleum oil plus 15% to 20% nonionic surfactant. Methylated Seed Oils (MSO) utilize chemically modified vegetable oils (soybean or canola).
• Mechanism: Softens and dissolves the waxy epicuticular wax layer on mature weed foliage, accelerating the internal uptake of systemic herbicides.
• Operational Hazard: High risk of severe crop injury (foliar burn) when applied during hot, humid weather (temperatures exceeding 85°F / 29°C).

Buffers, Acidifiers, and Alkaline Hydrolysis

• The Alkaline Hydrolysis Threat: Many water sources (especially well water in limestone regions or treated municipal supplies) have a pH of 7.5 to 9.0. When susceptible pesticides—particularly organophosphates, carbamates, and synthetic pyrethroids—are mixed with alkaline water, the hydroxide ions ($OH^-$) chemically cleave the active ingredient molecule into inactive fragments, a process known as alkaline hydrolysis.
• Degradation Speed: Under alkaline conditions, active ingredients degrade within hours or minutes:
  • Dimethoate: Half-life is 20 hours at pH 6.0, but drops to under 45 minutes at pH 9.0.
  • Malathion: Half-life is 8 days at pH 6.0, but drops to 5 hours at pH 8.0.
• Acidifiers: Chemicals (e.g., citric acid, phosphoric acid) that lower the spray solution pH.
• Buffers: Buffering agents that lower pH and chemically stabilize the solution against further pH drift, maintaining an optimal target range of pH 5.5 to 6.5.

Drift Control Agents & Deposition Aids

• Mechanism: Long-chain synthetic polymers (polyacrylamides) that increase spray solution viscosity, binding fine mist droplets together into larger, heavier droplets.
• Impact: Drastically reduces the percentage of "driftable fines" (droplets under 105–150 microns in diameter) that cause off-target drift.
• Handling Rule: High-shear centrifugal pumps can mechanically shear polymer chains, degrading drift-control performance over extended agitation.

Anti-Foaming Agents (Defoamers)

• Suppress air entrapment and collapse foam caused by aggressive hydraulic agitation of solutions containing high surfactant or wettable powder loads. Adding a few drops before adding pesticides prevents tank overflows.

---

3. Physical vs. Chemical Incompatibility

When two or more pesticides, or pesticides and liquid fertilizers, are mixed in a single spray tank, incompatibility may occur:

+-----------------------------------------------------------------------------------+
|                         INCOMPATIBILITY CLASSIFICATIONS                           |
+-----------------------------------------+-----------------------------------------+
|         PHYSICAL INCOMPATIBILITY        |        CHEMICAL INCOMPATIBILITY         |
+-----------------------------------------+-----------------------------------------+
| • Visible failure to mix or remain mixed| • Invisible chemical reaction in tank   |
| • Curdling, gelling, mayonnaise sludge  | • Active ingredients deactivated        |
| • Layering, oil separation, precipitates| • New phytotoxic compounds formed       |
| • Clogs screens, pumps, and nozzles     | • No physical signs (looks normal)      |
| • Caused by improper order, cold water  | • Caused by chemical antagonism / pH    |
+-----------------------------------------+-----------------------------------------+

1. Physical Incompatibility: The ingredients physically fail to combine into a uniform, sprayable mixture. Manifests as:
   • Gelling, clumping, or curdling into a cottage-cheese or mayonnaise-like sludge.
   • Phase separation into distinct, unmixable liquid layers.
   • Crystalline precipitation settling to the bottom of the tank.
   • Result: Clogged intake strainers, damaged pump seals, plugged nozzles, and erratic application rates.
2. Chemical Incompatibility: A chemical reaction alters the molecular structure of one or more tank partners. No visible signs appear in the tank. Manifests in the field as:
   • Complete loss of pest control efficacy (antagonism).
   • Severe, unanticipated crop phytotoxicity (leaf burn, stunting, chlorosis).
   • Result: Total application failure and crop destruction despite a smooth, normal-looking spray mix.

---

4. The WALES / WALE Tank-Mixing Sequence

Adding formulations to the spray tank in the incorrect order is the primary cause of physical tank incompatibility. Applicators must follow the standardized WALES (or WALE) protocol:

Critical Rule: If water-soluble packets (WSP) are used, they must be added to clean water first (Step 2) and allowed to dissolve completely before adding ECs or oil-based products. Adding oil-based products before WSPs coats the PVA film, preventing the packet from dissolving and leaving intact plastic bags floating in the tank.

---

5. The Step-by-Step Jar Compatibility Test

Before mixing thousands of dollars of chemical products in a 500-gallon spray rig, applicators must perform a small-scale Jar Compatibility Test:

1. Use the Actual Carrier: Obtain a clean 1-quart glass jar. Add 1 pint (16 fl oz) of the exact water or liquid fertilizer carrier that will be used in the field.
2. Calculate Proportional Quantities: Add proportional amounts of each pesticide based on intended field application rates per 100 gallons:
   • For each 1 lb per 100 gallons of dry product (WP/WDG): Add 1 level teaspoon to the jar.
   • For each 1 pint per 100 gallons of liquid product (EC/SC): Add 1/2 teaspoon to the jar.
3. Follow the WALES Sequence: Add products individually in the exact WALES order, capping and shaking the jar gently for 10–15 seconds after each addition.
4. Observe Stand Time: Let the capped jar stand undisturbed for 10 to 15 minutes.
5. Evaluate Compatibility:
   • Compatible: The mixture remains smooth, uniform, and suspended, or separates slightly but redisperses completely with 2–3 gentle inversions. The mix is safe for the spray tank.
   • Incompatible: The mixture displays heat generation (exothermic reaction), curdling, sludge, crystalline settling, or layering that will not redisperse. Do not tank mix.
   • Testing a Compatibility Agent: If incompatible, repeat the test adding 1/2 teaspoon of liquid compatibility agent to the water before adding pesticides.

---

6. Exam Tips & Practical Field Scenarios

Exam Tip #1: Mixing Sequence Traps: Questions testing tank mixing will often ask when to add surfactants or emulsifiable concentrates. Remember: Dry formulations (WP/WDG) go in first; Surfactants and Soluble liquids go in last.

Exam Tip #2: Alkaline Hydrolysis Solution: When mixing organophosphate insecticides with spray water tested at pH 8.2, the correct procedure is to add a buffering acidifier to lower and stabilize the water pH to 5.5–6.0 before adding the pesticide.

Exam Tip #3: Crop Oil Concentrate in Heat: Applying a post-emergence herbicide with a Crop Oil Concentrate (COC) at 92°F under direct sunlight will cause severe foliar phytotoxicity because the hot oil destroys the protective leaf wax of the crop.